Seam depth indicator



SEAM DEPTH INDICATOR 4 Sheets-Sheet l Filed Dec. 14, 1949v DA. smv SMNNm. TOL l MME m vl. HG \\WN C.O T T Mm V L mm /U mm. QN www i oFJmoWZNDQME) rw.: 90 mmjav ozmnomml \m.h rm.: mzjo...) moOIPdo hw .Mlh N Nov24, 1953 w. c. HARMON ETAL 2,660,704

SEAM DEPTH INDICATOR Filed Dec. 14. 1949 4 Sheets-Sheet 2 42 (lo 2l 'UHU"w" Nov 24, 1953 w. c. HARMoN Er/u. 2,660,704

SEAM DEPTH INDICATOR Filed Deo. 14, 1949 4 Sheets-Sheet 3 130 rf 120 11o10? if@ 2 I Joli L 201 +200@ m WTA INVENTORS;

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Nov 24, 1953 wl C, HARMON ETAL 2,660,704

SEM DEPTH INDICATOR Filed Dec. 14, 1949 4 Sheets-Sheet 4 2u 212 ZIO 230Z l' I m 22g 231 1 my 215 V 232 214 Hff ggz 24 225 rf Z l,fglxfv 223%hzwy ' INVENTORS. WLLAiV CHARMON Ama Patented Nov. 24, 1953 UNITEDSTATES PATENT OFFICE SEAM DEPTH INDICATOR Application December 14, 1949,Serial No. 132,974

- 22 Claims. (Cl. 324-34) Our invention relates to the detection ofiiaws in metal articles. The invention is particularly suited to thedetection of superiicial flaws in billets or metal shapes; such aws asseams, cracks, laps or slivers. The surface characteristics of steelbillets, for example, are such that the detection of defects isdifiicult. Supercial cracks may be almost, if not entirely, invisible tothe naked eye. Moreover, the surface appearance of a defect oftenaffords no indication of the depth and, therefore, the significance ofthe defect.

Our invention is embodied in a detecting device for ilavvs which may beregulated to indicate flaws above a predetermined depth and thereby callattention only to those defects which are of suicient magnitude to benoteworthy.

The preferred embodiment of the invention in apparatus for testing forseams and other flaws possesses many advantages and involves many novelfeatures. The instrument in which the invention is embodied is bothexceptionally sensitive and quite stable. The instrument is portable,and the search unit which engages the material being tested is of suchnature as to maintain the proper spatial relation to the work. Thesearch unit includes an indicator which is immediately energized when aliaw is encountered, so that the operator is immediately informed of thepresence and location of the flaw. The instrument can be adjusted forvarious minimum depths of avvs to which it will respond, and will retainthis calibration accurately. Only a small portion of the apparatus needbe at the point where the examination is being conducted and this partmay be connected to the less readily portable portion of the apparatusby a relatively long conductor. rlhus, the instrument may beconveniently used either over a substantial area or in cramped quarters.The instrument requires no tuning or calibration after initialadjustment except the setting for seam depth, which may be rapidly andeasily accomplished by the use of a test sample containing seams or cutsof known depth.

The primary object of the invention is to improve the detection of awsin metal articles and more specifically to increase the accuracy,reliability, and convenience of such detection. Subsidiary objects ofthe invention relate to the provision of desirable operatingcharacteristics, as outlined in the foregoing summary of the advantagesof the invention. Many other objects and advantages of the inventioncontributing to the realization of the primary object thereof will beapparent to those skilled in the art from the detailed descriptionherein of the preferred embodiment of the invention.

Referring to the drawings, Fig. l is a block diagram of the electricalsystem of the invention; Fig. 2 is a perspective view illustrating thepreferred physical arrangement of the system; Fig. 3 is a side elevationof the hand search unit; Fig. 4 is a plan view of the same; Fig. 5 is anenlarged side elevation of the search coil; Fig. 6 is a bottom view ofthe same; Figs. 7A and 7B constitute a circuit diagram of the invention;and Fig. 8 is a view of a device for testing rounds.

By Way oi introduction to the detailed description of the preferredembodiment of the invention, the operation thereof may be brieydescribed. lt involves moving a search coil closely over the materialbeing tested. This coil is energized by an oscillating vacuum tube andgencrates currents in the steel or other metal being tested. When a flawin the metal is encountered, the loading of the search coil isdecreased, and, therefore, the output of the oscillator increases. Thisvariation in oscillator output ultimately controls a signal lightmounted in the Search unit, which iiashes Whenever a defect isencountered of magnitude equal to or greater than that for which theinstrument is set. rFhe electrical system by which this result isobtained involves many features contributing in securing accurate andconsistent operation of the device.

In the preferred physical embodiment of the apparatus, iilustrated inFig. 2, the hand search unit ie is connected by a short shieldedflexible cable H to a portable remote unit i2, with a shoulder strap i3,which contains the oscillator to energize the search coil and additionalcircuits by which the variations in oscillator output are amplified andthe output impedance of the unit is lowered so that the remote unit maybe at a considerable distance from the major part of the electricalsystem. The unit I2 is connected by a multi-conductor shielded cable It,which may be at least a hundred feet in length, to a main electronicchassis i5. The chassis l5 contains the greater part of the electroniccircuits apart from the power supply, which is housed in a power supplychassis i6. The power supply chassis l5 is energized from the usualalternating current supply through a cable il and is also provided Witha flexible ground lead i8 by which the power supply chassis is groundedwhen in use. The main electronic chassis is energized through a powercable i9 from the power supply.

The hand search unit l0 (Figs. 3 and 4) comprises a Bakelite handle 2|to one end of which is connected to the plate and screen grid of thetube, which are tied together. The opposite end of coil 63 connectsthrough lead 00 to the cathode of oscillator control tube |3|. The gridwinch ing 64 of the transformer is connected at one end to the grid ofthe tube and the other end, which is connected to ground, and the center'tap of the grid coil are coupled to the search coil 20 through theflexible lead li (Fig. 1). The grid coil contains 16,000 turns in thepreferred embodiment, and the grid and plate windings areelectrostatically shielded from each other. With this coil, thefrequency of operation of the circuit is approximately 15,000 cycles persecond; however, the constants ina-y be Varied to obtain frequencies ofoscillation if desired. Windings 02 and 63 are connected by a platecircuit bypass condenser 05 and the plate coil is connected through alead 00 to the feedback circuit, will be described presently. Thefilament of the tube 0| is energized from the 200 volt plate supplythrough a resistor i nnd is bypassed by condenser 08. The resistor r isphysically mounted in the main chassis and supplies the ilarnent of tube0| through o, lead 12 which is carried through the flexible cable irl.Bypass condensers are mounted the main electronic chassis. Ahalf-megohni resistor i3 couples the oscillator to the highefrequencyamplifier circuit E0 and prevents undesired interaction between 'theoscillator and amplifier tubes.

The action of the oscillator circuit 60 is based. upon known principlesof feedbacl; between the plate and grid circuits which require noexplanan tion. The search coil 20 supplied by the oscillator induceseddy currents in the specimen Defects increase the length of the eddycurrent path and thus decrease the load or" coil 20 and therefore theload of the tube 0 i. The amplitude of oscillation of tube El is thusincrei ed when defects are encountered, and the deeper 'the seam, thegreater the unloading effect and consequent increase in oscillationamplitude. These defects cause rapid changes in the loading' of theoscillator' as the search coil passes over them. Other less rapidvariations due to surface conn ditions and curvatures of the surface,for ey.- ainple, are compensated through the feedback circuit whichcontrols the energization of the plate of the oscillator tube 0i throughthe lead t0.

The first high-frequency amplifier circuit l5, a high-gain circuit,comprises a type GAUE pentode voltage amplifying tube the of which isenergized by the oscillator 'through sistor 73. The cathode andsuppressor grid tube 'l0 are supplied from the ground line through acathode bias resistor 'il bypassed by condenser 18. The plate of tube isenergized the 200 volt supply through a plate circuit decouplingresistor 'i3 and condenser 80 and load resistor 79. The screen grid oftube is ener-- gzed from the 200 volt supply through dropping resistor8| and bypassed by condenser The circuit l5 amplies the output of theoscillator in a conventional manner.

Heater circuits for this tube and for other tubes in the device excepttube 0i are not illustrated in the circuit diagram, since thearrangement of heater circuits is conventional and need not be explainedto those skilled in the electronic art. The plus 200 volt supply isobtained from the power supply chassis I6 through the flexible cables I4and l0. The power supply system will not be described since it may be ofany suitable type known to those skilled in the art. For best dperformance, the' power supply should embody regulators by which thevoltage is held substantially constant.

The amplifier output is of high impedance and would be unsuitable fortransmission to the remote main electronic chassis. The cathode followeroutput circuit is of low impedance and permits the output of theportable unit to be fed through the long cable Ill. The cathode followercircuit comprises a triode amplifying tube 36 of the 6C@ type, the gridof which is coupled to the amplifier l5 by condenser 03 and grid currentlimiting resistor 92. The plate of the cathode follower tube is directlyconnected to the 200 volt line and is bypassed by condenser Sl. Thecathode is connected to ground through a 10,000 ohm load resistor BSwhich is preferably located in the main electronic chassis. Grid bias isestablished by a voltage divider composed of resistors and 00, of 30,000and 200,000 ohms respectively, connected between the cathode and ground,the junction between these resistors being connected through a grid leak0| and the grid current limiting resistor il?. to the grid of tube iii?.The voltage divider provides the best value of grid bias for tube S0. Inaccordance with the wellknown principle of operation of cathode followertubes, the cathode of tube 86 follows the uctuations of potential of thegrid of the tube.

The cathode is connected by a lead to the main chassis to a condenser 03which couples the cathode follower output to a potentiometer by whichthe output of the cathode follower is variiably coupled to the feedbackamplifier |00. The

feedback amplier circuit |00 comprises one half of a SSN? double triodelili, the half employed in the feedbackcircuit being identified aslilla. rEhe cathode of |0|c is energized through a bias resistor |02bypassed by condenser |00. The grid is energized from the potentiometerS5, which controls the amount of feedback, through a grid currentlimiter |00. The plate of lala is energized through a plate circuitdecoupling resistor |05 and load resistor |00, plate circuit decouplingcondenser |07 being connected between the plate load and ground.

rThe output of amplifying circuit 00 is coupled to the feedbackrectifier lill by a coupling condenser |03 and a current limitingresistor I The rectiiier circuit comprises a double diode l i2 of theGHG type with the two diodes connected in parallel. The plates areconnected to the current limiting resistor l i and the cathodes toground through 0.1 rnfd. condenser llt.

The voltage across the rectiner l i2 impressed on the time-delay circuit20. The circuit E20 is so designed that the output ci circuit |20 whichcontrols the operation of the feedback circuit is rapid enough tocompensate for drift the circuits, changes in surface texture, and othervariations which occur at a relatively slow rate. The lter networkcomprises a 2 megohn'i resistance E25 connected across the tube H2 and a2 megohm resistance i and 0.02 rnfd. condenser i in series between theplates of tube liz and ground,

The junction of resistor E22 and condenser |23 is connected to theoscillator amplitude control circuit |30, specically to the grid of thetriode |3|, which may be a @J5 tube. The cathode of tube |3| isconnected to ground through a 10,000 ohm potentiometer |25 which forms acathode resistor for the tube. The cathode of the oscillator controltube |3| is also connected through line 60 tothe plate circuit of theoscillator tube 6|. The plate of tube |3| is supplied through a resistor|33 and bypassed by condenser |34. The variable center tap |26 ofpotentiometer |25 is connected to the resistor |2| and the cathode ofthe rectifier ||2.

We may now consider .the operation of the feedfack circuit. When tube 6|is oscillating, the swing of the grid voltage of tubes 6| and 16 isamplified by the first high-frequency amplifier 15, the cathode follower85, and feedback amplifier |00, and the voltage output of the feedbackamplifier is approximately proportional to the .amplitude of theoscillations. First, `assuming that the amplitude of .oscillation iszero, the amplitude of the plate voltage swing of Ithe feedback ampliierwill likewise'be zero. Under these conditions `we may also assume thatthe oscillator control tube |3| has arrived at a steady state ofoperation with constant plate current and, there fore, that the voltagedrop in the resistor |25 is constant. The potential of the tap |26 ofpotentiometer |25 is likewise xed at a positive value less than that ofthe cathode of tube |3|. The condensers |I3 and |23 will be charged tothe potential of the tap |26 and no current will be flowing in resistors|2| and |22. There would, thus, be no potential across the rectifiertube H2 and capacitor |08 would .be charged .to the difference betweenthe potential of the plate tential level. This alternating `voltage istransmitted vto the plates of the rectier H2 through the condenser |08and resistor When the plates of the rectier swing positive, current isconducted through the rectifier and potentiometer |25 to ground.Y Whenthe rectifier plates are negative to the cathode, no current flows.Therefore, the average potential of the plates ||2 becomes more negative'and the 4potential difference across condenser |08 increases. Condenser|23 therefore partially discharges and the grid of the oscillatorcontrol tube |3| becomes more negative. At each positive swing of theplate. of; ||a., the condenser |00 receives an additional increment ofcharge, condenser |23 further discharges, and the grid potential of tubetat isl made more negative. Ultimately, under a steady condition ofoscillation, the maximum positive potential of the rectifier tube platesequals they potential of point |26, and the plates swing. negativelyYfrom this value, the amplitude of the swing being determined by theamplitude of' oscillations of the tube 8|. Thus, thel greater' theoscillation of the tube 0|, the lower the average value ofl potential ofthe plates of the rectifier and, thus, of the grid of the control' tube|31. When the grid of control tube |31 becomesamore negative, the tubeconductsA less. current, the drop through resistor |25 decreases andtheen.- ergizing voltage of theipl'atel ooscillator Gili diminished. Thus,the circuit described tends` to maintain the level of oscillation of'the tube 0|: at a constant value, but operates with a delayed orintegrating response, since the responseld'eponds upon charging ordischarging' condenser |123.' through .a high: resistance.

-When there is a transient disturbance of the oscillator due to a flaw,oscilla-tion amplitude-increases before-any sig-nicant compensationoccurs. The increase initiates a compensating ace tion in the controlcircuit |301whi'ch is reversed about kc.

. current limiting resistor' |62".

after the flaw is crossed by the detector. This action of the controltube causes a transient decreast in the amplitude of oscillation afterthe naw is crossed until the system again returns to normal. Thus, eachiiaw causes a transient increase in oscillation level immediatelyfollowed by a transient decrease.

As will be apparent, if the average level of oscillation decreases, thecondenser |23 will be charged additionally, the grid of the control tube|3| will be more positive, more current will flow through this tube, anda greater plate voltage will be supplied to the oscillator. Thus, thesystem so far described provides a very stable selfregulatingoscillating system which is sensitive in response to defects, but isself-regulating -to accommodate drift in circuit constants or changes inthe overall character of the surface tested. The above discussion hasassumed that the tap |26 of the potentiometer |25 is fixed, which is thecase in normal operation. This tap provides a direct control of thenormal grid potential and thereby the conductance of tube |3|. It thusserves as a convenient adjustment of the average oscillator amplitude ofthe system by determining the normal plate voltage of the oscillator.

The output of the cathode follower circuit 85 supplies, in addition tothe feedback circuit prevously described, a circuit `for operating anoscillator amplitude meter and the circuit by which the indicator light39 is actuated. Excitation for both these circuits, as indicated in Fig.1, is taken from the cathode follower 85 through the potentiometer andthe second high-frequency amplifier |40 by which the signal strengthisin-f creased. The output of the amplier |40 is divided to control thetwo above-mentioned circuits.

The cathode of tube 86 is connected through the coupling condenser 93 tothe potentiometer |35, which is in parallel with the feedbackpotentiometer 95. The variable tap of potentiometer |35 is coupled tothe grid of a triode amplifier llb, which is one half of the doubletriode |0|, through a current limiting resistor |4|. The' cathode oftriode Ililb is biased by resistor |42` bypassed by condenser |43. Theplate of |0|b` is energized from the plus 200 volt supply through adecouplingv resistor |44, decoupling condenser |46 and plate loadingresistor |45; The highfrequency amplier |40' is thus a conventionalamplifying circuit; however, it is designed to provide adequateamplification abilities up to The plate of the triode |0Ib is coupledthrough condenser |.4`| to potentiometer |50, the other end of which isgrounded.

The variable tap of potentiometer |50 provides moans for adjusting thesensitivity of the osci'llator amplitude meter circuit which comprises(Fig. 1) meter ampliiier |60, meter rectifier |10, meter filter |80, andthe oscillator amplitude meter |923. The meter circuit ndicatesperform.-ance of the oscillator and `other circuits preceding the meter circuit.`It does not indicate defects. The meter ampliiier may'convenieritlyemploy one half l'l'a of a. dual' triode" v|6. Thegri'd of tr'od'e |5`|ais energized from thevari'a'fbl'e tap |5| of the ypotentiometer |50through a grid' The plate ofthe tube is connected through the primaryofa one.- toone ratio transformer |6`3`.and a plate circuit decouplingand voltage dropping resistor |64 to the 200 volt plate supply. A platecircuit decoupling condenser' |65 is connected between the transformerand ground. The cathode of |6|a is biased by resistor 166, bypassed bycondenser 161. The amplifier 166 acts as a conventional amplifier andits output is coupled to the rectifier |16 by the transformer |63.

The secondary of transformer |63 is provided With a grounded center tapand is coupled to the plates of a diode rectifier |11, the cathodes ofwhich are connected directly to the meter filter circuit |89. The filter|86 consists of a condenser 121, resistance 132, and condenser |83 in astandard arrangement to smooth the D. C. output of the rectifier. Theoutput of the filter is connected directly to the meter |90. The otherterminal of meter |26 is connected to the cathode of the amplier tube16|@ and thus to ground through the cathode bias resistor |66. Theoperation of the amplitude meter circuit is as follows: With nooscillations in tube 6|, the grid of tube liu is at constant potential,there is no alternating output from transformer |63 and, therefore,apart from incidental effects, no input to meter til. However, becauseof slight disturbances in the several stages of amplification, andbecause of Edison effect in the rectifier tube 11|, the meter would restabove zero if it Were connected to ground. The voltage drop in theresistance 62 is just sufficient to balance out these eects and causethe meter to rest at zero With zero oscillations. Obviously, the outputof the amplifier |66 and rectifier |16 is proportional to the amplitudeof oscillation of tube 6|, and the meter will indicate the amplitude ofthe oscillations. This meter is installed in the main electronic chassisso as to be Visible, but it is to be understood that the meter does notindicate defects, and is not necessarily calibrated to read in specificunits. The meter indicates the average amplitude of oscillation of tube6|, and thus indicates the performance of the oscillator and itsamplifying, feedback and control circuits. An abnormally high or lowreading of the meter indicates trouble in these circuits. rllhe normalreading of the meter |96 may, of course, be adjusted by thepotentiometer tap 15|. The very slight current in the circuit of themeter |96 has no signicant effect on the bias of tube 161:1.

The remainder of Fig. '7A and Fig. 7B illustrate What may be termed theindicating circuity the elements of which comprise the last two columnsof Fig. l. The purpose of this part of the systeni is to amplify theflaw pulses, eliminate spuri- -ous pulses, provide an adjustment for theminimum depth of flaw to which the device will respond, and provideenergy for the operation of 'the indicator lamp 36. The nature of thiscircuit will be most clearly understood from the following detaileddescription.

The output of the second high-frequency am- `pliiier 1136 is directlycoupled to the third highfrequency amplifier 2do which is the rst unitof the indicating circuit, and is a conventional :amplifying circuitdesigned to handle frequencies up to 30 kc. The plate of triode Ilb iscoupled to the grid of triode llb through the condenser Mi and resistor266. rEhe cathode of |6|b is biased by resistor 251, bypassed bycondenser 202.

The plate of the tube is coupled through the primaryof the one-to-oneratio transformer 203 t a plate decoupling circuit comprising resistance26d and condenser The center-tapped secondary 2|1 of the transformer 20Ssupplies the demodulator-clamper circuit 2id (Fig. 7B), which comprisesa 56 double .diode rectifier tube 1212 serving as a demodulator and asimilar tube 213 acting as a clamper. The demodulator 252 eliminates thehigh frequency oscillations, but preserves the modulating pulsesresulting from cracks and other defects. The clamper diode 213 energizedfrom the demodulator through a filter, preserves a zero reference levelabove which the pulses due to defects appear.

rlhe secondary Winding 2| is connected to the plates of rectier 212 andthe center tap of 2|| is coupled through a capacity-resistance filtercomprising resistor 2| 4 and condensers 216 and 2 |'1 to a load resistor212. The output of the demodulator tube 2li! is coupled by condenser 2|5to the cathodes of the clamper tube 213 is parallel. The anodes of tube2 |3 and the cathodes of tube 2i2 are grounded. As will be apparent,negative pulses are shorted by the tube 2|3, but positive pulses, whichare not conducted by the tube, are passed to the succeeding circuits.The normal potential of the cathodes of tube 213 is zero, but they swingpositive when a signal is received, In this Way a constant potential ismaintained at the input to the low frequency amplifier 226 except when apositive pulse from the detection of a iiaw is received.

An increase in amplitude of the oscillations drives the center tap ofwinding 21| more negative and a decrease in oscillations makes it morepositive. Thus, the positive pulses transmitted to the low-frequencyamplifier 22d are those resulting from the decrease in oscillationamplitude, which pulses have been found to be slightly stronger and moreconsistent than those due to the original increase in oscillations.

The first low-frequency amplifier 222 is a conventionalresistance-capacity coupled pentode circuit except that the cathode biasresistor is not bypassed by a condenser, in order to maintain thegreatest possible stability, and the circuit values are chosen so thatpositive input pulses will be amplified to the best advantage. The10W-frequency amplifier 22S comprises a 6SJ7 'pentode 22|, the grid ofwhich is coupled through a limiting resistor 222 to the cathodes of theclamper 213 and is grounded through the grid leak 223. The cathode oftube 22! is biased by resistor 22d, and the plate is energized from a250 volt positive supply through decoupling resistor 225, decouplingcondenser 22's', and plate load resistor 226. The screen grid isenergized 4from the 250 volt supply through a voltage dropjping resistor222 and is bypassed by condenser 222. The amplifier circuit 22u iscoupled by condenser 236 to a sensitivity control attenuating circuitcomprising voltage dividing resistors 231 and 232 and potentiometer 235.The potentiometer 235 adjusts the overall amplification of the lowfrequency amplifying circuit comprising the first 10W-frequencyamplifier 22d, the Wein bridge Vfilter 245, the low-frequencyamplifier-clamper 25), and the biased amplifier-clamper 216, and thusdetermines the threshold of response of the indicator. Thispotentiometer is mounted in the chassis I5 for convenient adjustment ofthe threshold.

The output of the low-frequency amplifier 220 is fed throughpotentiometer 235 and isolating resistor' 236 to a modified V/ein bridgerejection filter 24U which is employed to eliminate any 60- cyclevoltage which may have gotten into the amplifying circuits. The bridgecomprises 0.01 mfd. condensers 2M and 242 in adjacent legs of the bridgeand variable half-megohm resistors 243 and 2M, which are adjustable inunison, in

l1 the other leg of the bridge. The diagonal oi the bridge consists 01 a0.02 mid. condenser 245 and `a 250,000 `ohm variable resistor 24B, thecenter point `between the elements 245 and 240 being grounded, The4output of the Wein bridge -is taken from the `opposite corner from the`-input and is directly 4coupled to the low-frequency amplifier-dampercircuit 250.

This circuit .comprises rst a low-frequency amplifier which is aconventional circuit except that, to maintain the `greatest possiblestability, `the cathode fbias resistor is not bypassed with a condenser,.and vthe -circuit component values are so chosen .that negative .inputpulses are amplitied t best advantage. VThis amplifier comprises a GSJ'?pentode amplifier tube 25|, the grid of which, Vas stated, is energizeddirectly from the output of ythe Wein bridge` The cathode of the Vtubeis connected .to .ground through the bias re- :sistor 25,2. rEhe plateis energized from the plus 250 volt supply through the decouplingresistor .253, load .resistance 254, `and decoupling con- .denser 255.The screen. grid is energized through a voltage-dropping resistor 256and is bypassed 'by condenser 251. As will be seen, fthe circuit 'issimilar to that of the rst low-frequency amplier v220. Thenegativepulses on the grid of the amplifier v2 5| create positive pulses in theoutput, which are transmitted by a coupling condenser `2 58 4to thecathode of a clamper diode 259, half of a GHG tube. A half-megohmresistor 260 is `coupled between the cathode of diode 259 and ground andserves asa leak for the condenser 258, maintaining the cathode of 259normally at zero 'potential. Since the `plate of '259 is grounded, thepositivecutput pulses of tube v25| are transmitted, but any negativevoltage is short-circuited. Short positive pulses in the output of theamplier 250 are transmitted to the grid of the amplier tube 21| sincecondenser 258 has not suIicient time to discharge appreciably throughthe grid leak during these pulses.

The biased amplier-clamper 210 includes the triode 21|, which amplifiesonly positive input pulses which exceed a given level, and thus acts tonullify or block minor irregularities from the succeeding circuits. Thesensitivity level of the tube 21| or, in other Words, the minimum pulseto which it will respond, is controlled by varying the cathode bias. Thecathode bias of tube :21| .is derived from a voltage divider circuitbetween the plus 250 volt supply and ground comprising a resistor 212and potentiometer 213, the tap of which is connected to the cathode. Byvarying the position of the tap, the bias of the cathode of 21| can beadjusted to any desired value from zero to about 20 volts, so that thetube 21| is cut 01T at zero grid bias and at all levels of signal up tothe level which it is desired to transmit. The plate of triode 21| isenergized through the plate circuit decoupling resistor 214 andcondenser 215 and plate load resistor 216. The plate circuit is loadedby a 0.1 mfd. condenser 211, one side of which is grounded. The outputof biased ampliiier 21| is fed through coupling condenser 218 to theplate of a diode clamper 219, the cathode of which is grounded. Sincethe tube 219 does not conduct the negative output pulses of tube 21|, itpasses these to the output of the biased amplifier-damper circuit. Asillustrated in Fig. l, this output may be coupled to a tape recorder 280which is not illustrated in detail, since it may be a commercialoscillograph. The elimination, by the circuits described, of minorpulses and incidental irregularities in the supply to the oscillograph.promotes fa clean V-record of surface de- Afects. As illustrated yinFig. 7B, 4the output ofthe Fig. 1, the rst element of the triggercircuit being a potentiometer 285 by which the sensitivityrof thetrigger circuit is-regul-ated. The output of the biased4amplifier-clamper 210 is connected through thehalf-megohm-potentiometer 285 to ground, the potentiometer being shuntedby half-megohm Nresistor Y286 to reduce the impedance of the outputcircuit.

IThe variable 4tap -of potentiometer 2845 is connected tothe grid oftriode 29| of the amplier circuit 290 through -a gridV current limitingresistor -2 The cathode `of the tube is connected directly Vto groundand the plate is energizedfrom the `250 volt positive -supply Aby plateload resistance 293, decoupling resistance 294, and decoupling condenser2 95.

VDiodes 259 and 219 may be in the same envelope, and simiiarly withtriodes 21| and 29|. v 'Ifhe .plate ot the amplifier tube 29| isconnected tothe anode of agasgtrigger `tube 30| of the trigger tubecircuit 300. Tube 30 |,cf the VR15 type. Yis connectedtc yground!through resistor 302 and condenser 303 in parallel. KTube 29| isnormally conducting and the .potential at the plate of 29| isapproximately v25 volts, which is insufficient to trigger .gas ytube30|. However, the pulses on the grid of 29| are nega-tive and reduce theconductivity of the tube sot-hat 'the plate voltage increases. Pulsesabove the desired level for which the circuit is adjusted iire vthe gastube 30|, thus charging the condenser V3'0'3 which is preferably 0.5mfd. As soon -as the Apulse terminates, 'the potential of the .plate of29| decreases so that the trigger tube is extinguished and ceases toconduct. The setting of the slider on potentiometer 205 determines thepulse level at which the circuit triggers. Resistor '302 is preferably4,000 ohms. The condenser 303 vand resistor 302 act to lengthen theIpuls'e so that the output of the gas trigger tube will be of sufficientduration to produce a square wave pulse of sufficient duration in theoutput of the trigger' pulse amplifier 3|0 to operate relay 35|. Thetrigger pulse amplifying circuit 3|0 is provided primarily to eliminatespurious indications by providing a strong pulse oi' approximatelyconstantstrength for each ring of the gas tube 30|. vThe output pulsesYfrom the trigger pulse amplifier 3|0 drive the relay control circuit330, which, in turn, operates the quick acting relay 35|.

The trigger pulse amplifying circuit 3|0 consists of twostages o'famplification employing a dual tiiod high nu 6SL7 tube 3| i. The grid ofthe triode 3||d which performs the rst stage of the amplification isenergized from the gas trigger tube through a grid current limitinginput resistor 3|2. The cathode of this triode is biased by a voltagedivider vcomprising resistors 3|3 and 3|4 connected between the plus 250volt line and ground, resistor 3|3 being of 5,000 ohms and resistor 3|4a cathode bias bleedr of one-half megohm. This divider supplies a largerthan normal cathodevbias to` triode 3| la soV that 3| la amplies onlythat portion of each trigger pulse which exceeds a given level, thus'maintaining the duration of the square wave output pulse at the desiredvalue and leaving the tube insensitive to circuit disturbances. Theplate 3| la is energized through the decoupling resistor 3|5, plate loadresistor SIS, and decoupling condenser The plate of 31m is coupled tothe grid of 3l lb through a 0.1 mid. condenser tit and a grid currentlimiting resistor 3H?. Triode 3l ib is provided with a grid leak 32d.The plate of 3Hb is connected in a similar manner to that oi 3l la byresistors 32! and 332 and condenser 323. A .0003 mid. condenser 324 isconnected between the two plates of tube 3i! to suppress parasiticoscillations. The cathode of triode 3l Ib is connected directly toground. The positive input pulses when fed through the two stages of thetrigger pulse ampliier 310 give rise to positive output pulses which areor substantially equal amplitude, since the amplification of the twostages is great enough to produce a high degree of limiting on allpulses which appear across condenser 303. The output of amplifier Sie istherefore a substantially square wave pulse, the pulses beingsubstantially identical in amplitude.

Amplifier circuit Sill is coupled by condenser 325 to the relay control.tube circuit 333, the condenser 325 being connected to the grid of thetype SVS pentode amplifying tube 33! through a current limiting resistor332, a grid leak being provided in the grid circuit. The relay controltube is normally nonconducting and is driven to maximum conductance bythe pulses from the trigger pulse amplier for a deiinite time. Sincethese pulses are substantially square, tube 33i acts like a switch whichis quickly snapped on and then off. Resulting changes in the potentialof the plate of tube 33! are applied to the quick acting relay 35i,producing extremely rapid and reliable operation of the relay. Thecathode and screen grid of tube 33! are energized through a voltagedivider comprising resistors 333, 334, and 335 connected between thepositive supply and ground to provide correct voltages for the cathodeand screen grid. The cathode resistor is bypassed by condenser 33S. Theplate is supplied through decoupling resistance 33'! and load resistance333 and decoupling condenser 339.

The coil 352 of the relay 35i is energized from the power tube 33!through condenser 3M, and is shunted by resistor The contacts of thisrelay are normally open and the relay is normally deenergized. When tubeconducts and energizes the relay, it closes an energizing circuit forthe indicator lamp 33 and for an indicator light mounted in the mainchassis. The circuit by which the indicator light 39 is energizedcomprises a bleeder lter circuit connected to inovable contact 35'! ofthe relay, including resistor 354 connected to the positive line andresistor 355 and condenser 355i connected to ground. The fixed contactis connected through a current limiting resistor 358 to the neonindicator light 39 in the hand search unit. It is also connected throughcurrent limiting resistor to a neon indicator 360 in the main electronicrlhe lead from the relay to the hand search unit passes through cablesit and H, in which it is shielded. The lead to the indicator lamp isgrounded at the main electronic chassis and the portable unit throughbypass ccndensers 3c! and 352, respectively.

Although the overall operation the indicating circuit should be clearfrom the foregoing, it may be reviewed brieily. An increase in theamplitude of oscillation of the tube el due to a defect increases thealternating output of the third high-frequency amplier 203. rThis outputis rectied and the normal output level is held at zero in thedemodulator clamper circuit 2li). Positive impulses caused by defectsare ampliiied in the low-frequency amplifier 220, the output of which isfed through depth control potentiometer 235 to the Wein bridge lter 240tuned to eliminate 60 cycle components. The iiltered signal is furtheramp-lined in the lowirequency ampliiier i, the output of which, apartfrom transmission of pulses, is held at zero by the clamper 259. Thebiased amplier Z'H rejects minor pulses and disturbances and passes theamplified defect pulses by way of a iinal clamper Zie to the recorder23u, if one is employed, and to the trigger circuits for the defectindicating light.

Pulses entering the trigger circuit are amplified by the amplier Zeilwhich controls the operation of the gas di charge tube 385, causing thetube Btl to hre at each pulse which exceeds a critical level. When thegas tube iires, it charges condenser 3&3, the Voltage pulse attendingthe charge and discharge of this condenser controlling the high-gaintwo-stage trigger pulse amplier BIB, which, in turn, drives the relaycontrol power tube circuit 330. The output of this circuit energizes thequick-acting relay i which iiashes the neon indicator 35.

The several potentiometers in the system, except the potentiometer 235,serve to balance the sensitivity of the Various components of the systemand ordinarily need not be adjusted after the initial balancing oi thesystem unless circuit constants are changed, as by the replacement ofelements.

. The adjustment of the instrument for sensitivity of indications or, inother words, the minimum depth of flaw which causes the indicator lightto flash, is effected by potentiometer 235. The instrument as describedcan be set to indicate seams or cracks exceeding any given depth in arange oi from 0.005 inch to 0.125 inch by turning the adjusting knob 235on the main chassis. While, of course, the control knob forpotentiometer 235 could ce calibrated in terms of depth settings, thepreferred method of calibration of the instrument employs a section ofbillet, which may be represented. by the object 553 in Fig. 1, with .006inch wide slots of various known depths milled in the surface ci thebillet. Using this calibration sample, the instrument can be quickly andeasily set by moving the search coil over the crack of the minimum depthwhich it is desired to detect, and advancing the potentiometer slowlyfrom its zero setting until the indicator light Se ashes each time thesearch coil passes over the milled slot. The instrument will thenindicate seams of depth equal to or greater than the Calibrating slot.

The apparatus is then ready for use. A fiat surface may be inspected bymoving the search coil back and forth from edge to at right angles tothe seams or cracks so that the search coil passes over them. The coilshould be moved forward during each pass so that the entire .surface isscanned. Large rounds four inches or more in diameter, may be inspectedin a similar manner, rotating them after each scanning operation untilthe entire .surrace has been inspected. Rounds smaller than four inchesin diameter can be inspected by using a .search coil with guides toguide the coil smoothly around the circumference. Rounds may also beinspected by rotating them in front of a search coil and moving therounds or the coil longitudinally of the axis of the round. Many otherinspection procedures can be used, depending upon the material and thedefects.

An arrangement to facilitate the testing of rounds is illustrated inFig. 8. The device comprises a spring clip which may be attached to theSearch coil assembly to guide it along the surface of the round. Asillustrated in Fig. 8, the search coil assembly is mounted between thearms 22 of the handle assembly as previously described. The guidingmeans comprises a U- Shaped sheet metal bracket 4t2 which may bedisposed over the upper portion of the search coil assembly and clampedthereon and which iS provided with depending curved spring ngers 403adapted to engage the round The horizontal portion of the bracket isformed With upstanding flanges 4M and with an opening 4136 for the plug38. A pin itl n one arm of the bracket may fit into an opening providedin the Bakelite cube 26y and a thumb screw 408 threaded in the oppositearm of the bracket engages the opposite face of the cube 2S. The springfingers 453 are secured to the arms of the bracket by screws 40S. Theselingers normally tend to retain the search coil assembly in a centeredrelation to the round so that the center of the face of the sapphireshoe is tangent to the surface of the round. Since the fingers areflexible, they Will accommodate themselves to rounds 451| of Variousdiameters, and Where an extreme range of diameter is encountered anumber of centering means of diilerent proportions may be usedinterchangeably. The

centering means may be applied to or removed from the search unit bysimply uncoupling the plug 38, fitting the guiding member in place, andtightening the screw 408.

While for many purposes the portable type of instrument specificallydescribed herein is most suitable, it will be apparent that theprinciples of the system may be employed in installations where therelative movement between the object being tested and the search coil isperformed automatically or otherwise by mechanism. No changes in theprinciple of the system need be involved in utilizing the system in axed setup for detecting flaws in materials fed past the search unit.

As will be the lightness and compactness ot the search unit and L..rtable remote unit and 4very considerable area which may be covered bythese units without moving the main electronic chassis render theinstrument extremely convenient and suitable for many elds of use. Spaceis provided in the power supply chassis for storing the hand searchunit, the portable remote unit, and the connecting cables, except thelong cable I4. This cable may be coiled. The main electronic chassis andthe power supply chassis are fitted .vith handles so that the entireapparatus may be readily carried by the operator from one place toanother.

Although the instrument is portable, it is highly accurate andsensitive, stable in operation, and very easily calibrated. By providingthe defect indicator lamp in the search unit adjacent the search coil,the operator receives an immediate indication of the defects for whichthe instrument is set may mark them or take other appropriate action.

It will be apparent to those skilled in the art from the disclosureherein of the principles of the invention and the description of thepreferred 16 embodiment thereof that many modications of structure maybe made within the scope of the invention, which is not to be regardedas limited by the detailed description of the preferred embodiment.

We claim:

l. A device for detecting defects in metallic objects comprising, incombination, a search coil traversable over the surface of an object, anoscillator' energizing and loaded by the coil, a control circuitresponsive to the output of said oscillator including a delayed responseelement connected to the oscillator to oppose relatively slow variationsin the output thereof, means responsive to transient variations in theoscillator output for amplifying and rectifyng the oscillator output toprovide pulses indicative of flaws in the object, a pulse amplifiercoupled to said amplifying and rectifying means, and a flaw-indicatingdevice operated by the pulse amplifier, and responsive to pulses above apredetermined magnitude.

2. A device for detecting defects in metallic objects comprising, incombination, a Search coil traversable over the surface of an object, anoscillator energizing and loaded by the coil, a feedback control circuitresponsive to the output of the oscillator and having the output circuitthereof connected to the oscillator to oppose variations in output ofthe oscillator, the feedback control circuit including a delayedresponse element so that it does not respond immediately to transientvariations in oscillator output, means responsive to transientvariations in the oscillator output indicative of flaws in the object,and a signal device operated by the last-named means.

3. A device for detecting defects in metallic objects comprising, incombination, a search coil traversable over the surface of an object, anoscillator energizing and loaded by the coil, a feedback control circuitresponsive to the output of the oscillator and having the output circuitthereof connected to the oscillator to oppose variations in output ofthe oscillator, the feedback control circuit including a delayedresponse element so that it does not respond immediately to transientvariations in oscillator output, means responsive to transientvariations for amplifying and rectifying the oscillator output toprovide pulses indicative of flaws in the object, a pulse ampliercoupled to said amplifying and rectifying means, and indicating meansresponsive to pulses above a predetermined magnitude coupled to saidamplifying means.

4. A device for detecting defects in metallic objects comprising, incombination, a search coil traversable over the surface of an object, anoscillator energizing and loaded by the coil, a, feedback controlcircuit operative to oppose variations in output of the oscillator, thefeedback control circuit including a delayed response element so that itdoes not respond immediately to transient variations in oscillatoroutput, means responsive to transient variations in the oscillatoroutput for amplifying and rectifying the oscillator output to providepulses indicative of flaws in the object, relay means sensitive topulses above a predetermined magnitude controlled thereby, and a signaldevice operated by the relay means.

5. A device for detecting defects in metallic objects comprising, incombination, a search coil traversable over the surface of an object, anoscillator energizing and loaded by the coil, a. feedback controlcircuit responsive to the output or" the oscillator and having theoutput circuit thereof connected to the oscillator to oppose variationsin amplitude of output of the oscillator, the feedback control circuitincluding a delayed response element so that it does not respondimmediately to rapid variations in amplitude level of oscillator output,means responsive to transient variations for amplifying and rectifyingthe oscillator output to provide pulses indicative of flaws in theobject, a pulse ampliiier coupled to said amplifying and rectifyingmeans, a trigger circuit responsive to pulses above a predeterminedamplitude energized by the pulse amplifier to generate relativelyuniform signals in responsive to output of the pulse amplier, relaymeans controlled thereby, and a signal device operated by the relaymeans.

6..A device for detect-lng defects in metallic objects comprising, incombination, a coil traversable over the surface of the object, anoscillator coupled to the coil so that the oscillator energizes the coiland is loaded thereby, means coupled to the oscillator for amplifyingand re-ctifying the output of the oscillator, a delayed response circuitcoupled to the rectify/ing means, and means coupled to the delayedresponse circuit and the oscillator' to oppose variation in oscillatoroutput.

7. A device for detecting detects in metallic objects comprising, incombination, a coil traversable over the surface of the object, anoscillator coupled to the coil so that the oscillator energizes the coiland is loaded. thereby, means for amplifying and rectiiying the outputof the oscillator, a delayed response circuit supplied by the rectifyingmeans, and an oscillator control circuit controlled by the delayedresponse circuit and coupled to control the energiaation of theoscillator so as to oppose variation in oscillator output.

8. A device for detecting defects in metallic objects comprising, incombination, a coil traversable over the surface of the object, means onthe coil for engaging the object and guiding the coil over the surfacethereof, an oscillator coupled to the coil so that the oscillatorenergizes the coil and is loaded thereby, means for amplifying andrectifying the output of the oscillator, an integrating circuit suppliedby the rectifying means, and an oscillator control circuit controlled bythe integrating circuit and coupled to control the energization of theoscillator so as to maintain a substantially constant reference level ofoscillator output.

9. A device for detecting defects in metallic objects comprising, incombination, a search coil, a visible indicator, an oscillator coupledto the search coil and loaded thereby, an amplier boosting theoscillator output, a feedback control responsive to the oscillatoroutput and coupled to the oscillator and including a delayed responseelement to stabilize the oscillator output, and a control circuitcoupled to the amplifier and to the indicator to actuate the indicatorin response to rapid changes in amplitude of the oscillations.

10. A device for detecting defects in metallic objects comprising, incombination, a hand Search unit including a portable search coil; aportable unit comprising an oscillator coupled to the search coil andloaded thereby, and an amplier boosting the oscillator output; a mainelectronic unit coupled to the portable unit by a flexible cable andcomprising a feedback control adapted to stabilize the oscillator outputand a circuit responsive to rapid changes in ampli- Cai lil

tude of the oscillations; and means for exhibiting the presence ofdefects operated by the said circuit.

l1. A device for detecting defects in metallic objects comprising, incombination, a hand search unit including a coil, a shoe to maintain thesearch coil at a sed distance from the tested object, and a visibleindicator; a portable unit comprising an oscillator coupled to thesearch coil and loaded thereby, main electronic unit coupled to theportable unit by a flexible cable and comprising a feedback controladapted to stabilize the oscillator output and a control circuit for theindicator to actuate the indicator in response to rapid changes inamplitude of the oscillations.

12. A device for detecting defects in metallic objects comprising, incombination, a hand search unit including a Search coil, a shoe tomaintain the search coil at a fixed distance from the tested object, anda visible indicator; a portable unit comprising an oscillator coupled tothe search coil and loaded thereby and an ampliiier boosting theoscillator output; and a main electronic unit coupled to the portableunit by a liexible cable and comprising a feedback control adapted tostabilize the oscillator output and a control circuit for the indicatoradapted to actuate the indicator in response to rapid -changes inamplitude of the oscillations.

13. A unit for a naw detecting apparatus comprising, in combination, ahandle including a body portion and spaced strips extending from thebody portion, a search coil assembly comprising non-magnetic meanshaving a Work engaging surface for maintaining the assembly in apredetermined relation to the surface of the object being tested, asliver of magnetic material disposed so as to be maintainedsubstantially perpendicularly to the surface of the object and with oneend adjacent said surface, and a coil inductively coupled with thesliver, and pivots extending between the search coil assembly and thestrips for flexibly mounting the Search coil assembly on the handle andpermitting the nonmagnetic means to determine the relation of theassembly to the object.

14. A detector unit for a flaw detecting apparatus comprising, incombination, a handle including a body portion and spaced stripsextending from the body portion, a search coil assembly comprising ashoe of hard non-magnetic material having a plane Work-engaging surfaceand adapted to slide over a surface being tested, a sliver of magneticmaterial disposed so as to be maintained substantially perpendicularlyto the work-engaging surface of the shoe and with one end adjacent saidsurface, a coil inductively coupled with the sliver, and a non-magnetichousing for the coil and sliver, and pivots extending between thehousing and the strips for iiexibly mounting the search coil assembly onthe handle and permitting the shoe to determine the relation of theassembly to the object.

15. In a device for detecting variations in objects comprising a searchcoil, an oscillator coupled to the search coil, the oscillator having anoutput circuit and including a vacuum tube and electrodes in the vacuumtube, means supplying operating potentials to the electrodes, anamplifier coupled to the oscillator output circuit, and a feedbackcircuit coupled between the amplier and at least one of the electrodesto produce a potential variation in opposition to 19 `transient changesin the output circuit of Ithe oscillator.

16. The invention in accordance with claim 15, the said feedback circuitincluding 4a feed- .back amplifier coupled to the oscillator outputamplier and a control circuit coupled to the electrode and the feedbackamplier.

17. The invention in accordance with claim `15, the said feedbackcircuit including `a feedback amplifier, a control circuit connected toat least one of the electrodes and a delay device connected between thefeedback amplifier and .thecontrol circuit.

18. Afdevice for detecting variations in objects comprising a searchcoil, an oscilla-tor coupled tothe search coil, the oscillatorhaving lanoutput circuit `and including a vacuum tube and electrodes in the vacuumtube, means supplying operating potentials to the electrodes, a feed-Vback circuit `coupled between -the output circuit and at least one ofthe electrodesto produce a potential varia-tion in opposition ltotransient changes 4inthe output circuit ofthe oscillator, said feedbackcircuit including a delay `element for opposing rapid changes andrectifying means for rendering the circuit responsive only to variationsin oscillator output amplitude, an indicator 'device and means coupledbetween the output vcircuit and the indicating device for actuating theindicator device in vresponse to signals of greater `than predeterminedamplitude.

19. lThe invention in laccordance with claim 18, the last named meansvcomprising a trigger circuit adapted to pass `only impulses of greaterthan predetermined amplitude.

20. Theinvention in accordance with claim'l, thelast namedmeanscomprising a clamper and a trigger circuit coupled to theoutput of theclamper for amplifying impulses of greater than predetermined amplitude.

21. -The inventionfin accordance with lclam .118, the last named meanscomprisinga clamper, `an amplifier coupled to the `output of thelclamper, and a gaseous `discharge device coupled to .the output `of theamplifier, lthe discharge `device being -responsive'only to impulses ofgreater than predetermined magnitude.

22. Theinve'ntion in accordance with claim 115,

lthe said yfeed back circuit including a--rectier forrenderingfthecircuit responsive only`"to amplitude variations vof oscillatoroutput-anda delay device for Aopposing rapid-variationsinamplitude levelof oscillator\`o`ut'put Electronics, Jan. 1946, pages 10S-109, articleby D011.

Electronics, July 1949, `pages'80-83,`arti`cle by Urbach.

